1. Field
This patent specification relates in general to an optical recording medium, and more particularly to a phase change recording medium and the methods, optimally feasible for implementing read/write/erase operations at high recording velocities and attaining desirable overwrite characteristics and storage durability, among others.
2. Discussion of Background
Optical information recording media have recently come into use as viable information data storage and archival devices of large capacity. Of the optical recording media, rewritable optical discs have been in wide use in the area of computers, in general, and various home electronics instruments, as exemplified by CD-RW (compact disc-rewritable), PD (power disk), DVD(digital versatile disc)-RAM, DVD-RW and DVD+RW.
With such a trend of versatile use of rewritable recording media, it is a pressing need for the recording media to acquire high speed recording capabilities, among others, and this is especially true with rewritable CD-RW discs.
As to the high speed recording capabilities, an optical recording medium and a recording method therefor are discussed in Japanese Laid-Open Patent Application No. 11-115313. Although the recording medium is described therein using the parameters similar to those used in the present disclosure, the range of parameter change is different and the former is restricted to rather low recording velocities.
By extending the capabilities for CD-RW discs described in Orange Book (Part III, Vol. 2, Version 2.0), a recording medium is specified to have a specified upper limit of recrystallization linear velocity to meet recording velocities up to CD 8× speed (9.6 m/sec of relative speed). The recording medium described in the above disclosure, however, is considered to have difficulties such as deterioration in signal qualities after recording cycles especially at recording velocities of 9.6 m/sec or larger.
Similarly, another CD-RW recording medium is also specified, also in the Orange Book (Part III, Vol. 2, Version 2.0) to meet multi-speed recording, having a specified upper limit of recrystallization linear velocity. For the recording medium described in the above disclosure, however, no discussion is found with respect to recording at velocities 4.8 m/sec or larger.
As indicated earlier, phase change optical recording media typically exemplified by CD-RW and DVD-RW discs, have come into wide use because of rewritable capabilities, among their various advantages. However, these media have also drawbacks at present such as difficulties in recording, repeated recording (overwrite) and erasure, at various linear recording velocities (multi-speed).
As the demands from the market increase together with the improvements in disc drive capabilities, it becomes increasingly important to provide optical recording medium feasible for multi-speed operations. The media capabilities so far achieved are multi-speed operations of CD-RW discs at linear recording velocities ranging 1.2 m/sec to 4.8 m/sec. Therefore, it is highly desirable for the recording medium to achieve recording at higher linear velocities, and to meet another recent trend of higher density recording.
As to a further advantage of the rewritable recording media, direct overwrite capabilities can be cited, in which repeated read/write operations can be carried out without erase process step.
Because of exclusion of this erase step, however, signal qualities after overwrite steps tend to be affected considerably by the media properties prior to recording. Since this effect is particularly evident for the signal qualities after the first overwrite step, the improvement thereof is of primary importance. In addition, since recording steps proceed as thermal process, there gives rise to drawbacks such as deteriorating change after repeated recording cycles in materials property in the recording layer, to thereby worsen signal qualities.
The above noted trend of signal deterioration in recording media becomes more pronounced at higher recording velocities. When the composition of recording materials is selected so as to be suitable to alleviate the deterioration, storage durability has been found considerably deteriorated for the media.
Therefore, no rewritable optical recording media has been formed so far, having media properties that fulfill both requirements simultaneously, one being direct overwrite recording at high velocities and the other satisfactory storage durability.
In Japanese Laid-Open Patent Application No. 8-224961, for example, an optical recording medium is discussed including at least a recording layer of AgSbTe alloys, having an activation energy of crystallization of 3.0 eV or larger, and dielectric layers formed on both sides of the recording layer. Although improvements in storage life of the recording medium are indicated in that disclosure, satisfactory recording capabilities at higher velocities have not been achieved.
It may be noted herein the activation energy in this disclosure corresponds to that of the transformation from amorphous recorded marks to crystalline portions. The activation energy, however, was obtained for a recording layer, not incorporated into the recording medium, but formed between dielectric layers. This may gives rise an activation energy different from that in real recording layer configurations, as described herein below.
Similarly, in Japanese Laid-Open Patent Application No. 8-263871, another optical recording medium is discussed including a recording layer of AgInSbTe alloys with a specified composition, having an activation energy of 1.0 eV or larger. Although improvements in storage life of the recording medium are described in that disclosure, high speed recording has not been achieved.
It may be noted herein the above activation energy corresponds to that of the transformation from amorphous recorded marks to crystalline portions, which has been obtained for a recording layer incorporated into the recording medium, in which the activation energy value is clearly different from the value, 3.0 eV or more, disclosed in the above mentioned application '961.
According to Japanese Laid-Open Patent Application No. 9-248965, a further recording medium includes a recording layer having an activation energy (amorphous marks to crystalline transformation) of 3.5 eV or more. Although improvements in overwrite characteristics are indicated in that disclosure, its storage durability is not completely satisfactory.
In addition, in Japanese Laid-Open Patent Application No. 11 -129620, a recording medium includes a recording layer of AgInSbTe alloys having specified values of composition and crystallization temperature, in which improvements are indicated in its feasibility of satisfactory signal qualities, and high recording reliability achieved by an activation energy (amorphous marks to crystalline transformation) of 1.5 eV or larger. However, satisfactory signal qualities after repeated overwrite cycles are not always attained, in practice, and sufficient storage durability has not been attained either.
In this context, it may be noted herein below that recording capabilities at higher velocities are closely related to the crystalline state of the recording layer. For example, although recording materials, which tend to crystallize with more ease even at higher recording velocities, are preferably selected to achieve high recording speeds, these recording materials, in general, have low crystallization energies in recording layer.
For the low crystallization energy, the capabilities of recording, repeated recording (overwrite) and erasing at high recording speeds are generally improved, while storage durability decreases.
Storage life can be obtained from an activation energy for a single recording layer according to the method in the Application '961. Although the activation energy of transition from amorphous marks to crystalline portions may be obtained for a single, isolated recording layer by that method, an actual activation energy is considered different, in general, since the recording layer is actually placed in recording medium environment.
Accordingly, it is desirable to obtain more realistic energy value for a recording layer incorporated into the media, as in Japanese Laid-Open Patent Application No. 8-263871. The method in this application '871, however, is considered still insufficient for the accurate calculation of the energy because of its narrow range in consideration and insufficient number of actual deterioration (or failure) modes. In addition, the calculation for higher recording velocities is not feasible by this method.
Although several improvements have been made as mentioned herein above, there are several points yet to be improved with regard to satisfactory recording capabilities at higher velocities, signal qualities after repeated overwrite steps and storage durability. In addition, it is also desirable for the recording medium to meet another recent trend of higher density recording.
These may be achieved by further pursuing and optimizing the components of recording media in terms of recording materials, layer construction, together with their optical and thermal properties, among others.